There is a great new cheat with the celestial steed going around.
The first part of the cheat is to make sure you do not have flying enabled for the cheat to work. It makes it far easier to use this exploit.
Now you need to go to northrend. And make sure you dont have cold weather flying going on at the time.
Now mount up, then go in the water, look / aim down, Now right away hit space.
You should if done right be flying under water at upwards to 150% faster than before.